By definition, the rate of blood flow at any point in the cardiovascular system is expressed as the volume of blood that passes that point during a unit of time. Since an adequate blood supply is essential for all organs in the body, a measure of blood flow in a vessel that supplies a particular organ is of invaluable diagnostic importance. The blood flow rate is greatest in the pulmonary artery and the aorta, where the blood vessel leaves the heart. The flow at these points is called cardiac output.
There are numerous techniques available for the measurement of cardiac output. A well known one makes use of the principals of indicator (dye or thermal) dilution. In thermodilution systems, a small quantity of an indicator having a temperature typically lower than that of blood is introduced into the circulatory system and a thermistor is used to sense the temperature of the blood at a location downstream of the introduction site. As the cool indicator travels through the blood stream toward the thermistor location, the temperature of the blood temporarily decreases as its temperature is diluted by the indicator. Temporal variations in blood temperature resulting from the introduction of the indicator into the blood stream are sensed by the thermistor and are used to compute the blood flow rate or cardiac output. A plot of the changes in blood temperature as a function of time is referred to as a thermodilution curve, and the cardiac output is determined by measuring the area under this curve. A representative system for computing cardiac flow rates from thermodilution measurements is disclosed in U.S. Pat. No. 3,987,788 to Emil.
Alternatively, the indicator injected into the circulatory system may be a volume of non-toxic dye. The injected dye has a detectable light absorption characteristic different than that of blood and hence yields a densitometric output curve with the same generalized profile as a thermodilution curve. The variations in the amplitude of the detected data from either the thermodilution or the dye injection method may then be processed by the Stewart-Hamilton equation to yield cardiac output. The solution to the Stewart-Hamilton equation requires the integration of either the blood temperature change with time or the dye concentration change with time.
Much has been written about the inaccuracies of various integration techniques for estimating the area under the output curve. Inaccuracies stem, for example, from recirculation of the blood past the point of measurement, since the indicator is not dissipated immediately by the body, but may pass repeatedly through the heart, or from baseline drifts that lead to large errors if a long integration time is used. See, for example, U.S. Pat. No. 4,015,593 to Elings et al. However, error introduced into the calculation by spurious changes in blood temperature independent of the introduction of the injectate have been ignored. Thus, in conventional cardiac output computations, a start button is depressed to indicate to a computer that the injectate has been or will be made. The baseline for the integration calculations is established based on the temperature of the blood as of the time the button is depressed and held pending the integration of the curve. Since several seconds may elapse before the indicator or the bolus of injectate reaches the sensing device, the baseline may shift. Thus, establishing the blood temperature baseline as of this earlier time is inaccurate and may introduce considerable error into the computations since the integration is done with respect to the established baseline. The error introduced in such a system is proportional to the integral of the difference between the true baseline and the measured baseline at the start switch time over the integration time period.